Hemolysis in the spleen drives erythrocyte turnover.

Red pulp macrophages (RPMs) of the spleen mediate turnover of billions of senescent erythrocytes per day. However, the molecular mechanisms involved in sequestration of senescent erythrocytes, their recognition, and their subsequent degradation by RPMs remain unclear. In this study, we provide evidence that the splenic environment is of substantial importance in facilitating erythrocyte turnover through induction of hemolysis. Upon isolating human spleen RPMs, we noted a substantial lack of macrophages that were in the process of phagocytosing intact erythrocytes. Detailed characterization of erythrocyte and macrophage subpopulations from human spleen tissue led to the identification of erythrocytes that are devoid of hemoglobin, so-called erythrocyte ghosts. By using in vivo imaging and transfusion experiments, we further confirmed that senescent erythrocytes that are retained in the spleen are subject to hemolysis. In addition, we showed that erythrocyte adhesion molecules, which are specifically activated on aged erythrocytes, cause senescent erythrocytes to interact with extracellular matrix proteins that are exposed within the splenic architecture. Such adhesion molecule-driven retention of senescent erythrocytes under low shear conditions was found to result in steady shrinkage of the cell and ultimately resulted in hemolysis. In contrast to intact senescent erythrocytes, the remnant erythrocyte ghost shells were prone to recognition and breakdown by RPMs. These data identify hemolysis as a key event in the turnover of senescent erythrocytes, which alters our current understanding of how erythrocyte degradation is regulated.

Differential interaction between DARC and SDF-1 on erythrocytes and their precursors.

The Duffy Antigen Receptor for Chemokines (DARC) is expressed on erythrocytes and on endothelium of postcapillary venules and splenic sinusoids. Absence of DARC on erythrocytes, but not on endothelium, is referred to as the Duffy negative phenotype and is associated with neutropenia. Here we provide evidence that stromal cell-derived factor 1 (SDF-1), the chemokine that restricts neutrophil precursors to the bone marrow, binds to erythrocyte progenitors in a DARC-dependent manner. Furthermore, we show that SDF-1 binding to DARC is dependent on the conformation of DARC, which gradually changes during erythroid development, resulting in the absence of SDF-1 binding to mature erythrocytes. However, SDF-1 binding to erythrocytes was found to be inducible by pre-treating erythrocytes with IL-8 or with antibodies recognizing specific epitopes on DARC. Taken together, these novel findings identify DARC on erythrocyte precursors as a receptor for SDF-1, which may be of interest in beginning to understand the development of neutropenia in situations where DARC expression is limited.

In vitro detection of cholangiocarcinoma cells using a fluorescent protein-expressing oncolytic herpes virus.

Pathological confirmation is desired prior to high-risk surgery for suspected perihilar cholangiocarcinoma (PHC), but preoperative tissue diagnosis is limited by poor sensitivity of available techniques. This study aimed to validate whether a tumor-specific enhanced green fluorescent protein (eGFP)-expressing oncolytic virus could be used for cholangiocarcinoma (CC) cell detection. Extrahepatic CC cell lines SK-ChA-1, EGI-1, TFK-1 and control cells (primary human liver cells) were exposed to the oncolytic herpes simplex type 1 virus NV1066 for up to 24 h in adherent culture. The technique was validated for cells in suspension and cultured cells that had been exposed to crude patient bile. Optimal incubation time of the CC cells with NV1066 at a multiplicity of infection of 0.1 was determined at 6-8 h, yielding 15% eGFP-expressing cells, as measured by flow cytometry. Cells were able to survive 2-h crude bile exposure and remained capable of producing eGFP following NV1066 infection. Detection of malignant cells was possible at the highest dilution tested (10 CC cells among 2 × 105 control cells), though hampered by non-target cell autofluorescence. The technique was not applicable to cells in suspension due to insufficient eGFP production. Accordingly, as yet the technique is not suitable for standardized clinical diagnostics in PHC.

ICAM-3 activation modulates cell-cell contacts of human bone marrow endothelial cells.

The Ig-like cell adhesion molecule ICAM-3 is mainly expressed on human leukocytes and is involved in cell-cell interactions. Its expression on endothelium is observed during disorders such as Crohn's disease and in solid tumors. We found low but detectable expression of ICAM-3 on VE-cadherin-expressing cells from primary human bone marrow aspirates, i.e. endothelial cells, and on primary human endothelial cells from cord blood. Also, immortalized human umbilical cord endothelial cells and human bone marrow endothelial cells showed ICAM-3 expression. However, its function on human endothelium is not known. Surprisingly, activation of endothelial ICAM-3 by crosslinking with specific antibodies resulted in a drop in the electrical resistance of bone marrow endothelial monolayers. In line with this, immunocytochemical analysis showed a loss of endothelial cell-cell contacts after ICAM-3 crosslinking in HBMEC. Detailed biochemical analysis showed an association of moesin and in a later stage ezrin with ICAM-3 upon crosslinking in HBMEC. Moreover, ICAM-3 crosslinking induced the production of reactive oxygen species (ROS), which are known to be involved in the control of endothelial cell-cell contacts. In conclusion, we showed that ICAM-3 is expressed on human bone marrow endothelial cells and controls endothelial integrity via ROS-dependent signaling.

Regulation and kinetics of platelet-activating factor and leukotriene C4 synthesis by activated human basophils.

BACKGROUND: Allergic disease is the result of an interplay of many different cell types, including basophils and mast cells, in combination with various inflammatory lipid mediators, such as platelet-activating factor (PAF) and leukotrienes (LT). LTC4 synthesis by human basophils has been studied quite extensively. However, not much is known about the synthesis of PAF by human basophils. OBJECTIVE: In this study, we have made a comprehensive comparison between the kinetics of PAF and LTC4 synthesis, in highly purified basophils, activated with different stimuli or with combinations of stimuli. METHODS: Synthesis of PAF and LTC4 by human basophils was determined with commercially available assay kits. The basophils were activated with C5a, fMLP, PMA, allergen or anti-IgE, in the absence and presence of IL-3 and/or in combination with elevation of cytosolic free Ca2+ by the sarcoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin. RESULTS: Most stimuli were found to induce both PAF and LTC4 synthesis. PAF synthesis and LTC4 release were enhanced by preincubation of the basophils with IL-3 or by elevation of cytosolic free Ca2+ by thapsigargin. Incubation of human basophils with IL-3 alone or thapsigargin alone did not result in detectable synthesis of PAF and LTC4, whereas the combination of the two resulted in high amounts of PAF and LTC4 synthesis. Depending on the stimulus used, LTC4 release was 5-100-fold higher than PAF synthesis. In addition, PAF, but not LTC4, was transiently detected, probably due to PAF degradation. LTC4 and PAF synthesis was strongly blocked by inhibitors of cytosolic phospholipase A2, indicating that this enzyme is involved in PAF and LTC4 synthesis by activated human basophils. CONCLUSION: This study provides a first comprehensive comparison of PAF and LTC4 synthesis in highly purified human basophils, stimulated with a variety of stimuli.